Apatite geochronology of the Sept-Iles Intrusive Suite (Grenville province, Canada) reveals two magmatic episodes 55 Myrs apart related to plume arrival and subsequent rifting

Abstract

The Sept-Iles Intrusive Suite, Quebec, Canada, comprises several components, including the third largest mafic layered intrusion in the world. Until now, the age of the whole suite was estimated at 565 ± 4 Ma based on zircon U-Pb dating from a single sample. However, the Layered Series component consists of three Mega Cyclic Units (MCU) which may record distinct magmatic events. This study uses apatite and zircon U-Pb and Lu-Hf geochronology to constrain the emplacement age of samples from the top of MCU I and II. Apatite from MCU I yields U-Pb dates of ca. 620 Ma. Samples from MCU II, stratigraphically above MCU I, yield a homogeneous U-Pb apatite date of ca. 565 Ma, overlapping the Lu-Hf apatite date of 567 ± 14 Ma, and the U-Pb zircon crystallization age of 564 ± 2 Ma. Based on these new ages and geochemical signatures, we propose a revised nomenclature for the Layered Series of the Sept-Iles Intrusive Suite. The ca. 620 Ma unit is divided into the Rivière Sainte-Marguerite unit (RSM; former MCU I) and Rivière des Rapides unit (RdR; former MCU III), the high (Gd/Yb)_N ratios in RSM cumulus apatite indicating partial melting at depth related to the Central Iapetus Magmatic Province mantle plume. The Lac au Castor unit (LC; former MCU II) was injected as a sill at ca. 565 Ma, with lower (Gd/Yb)_N ratios in cumulus apatite reflecting decompression melting of shallow asthenospheric mantle during subsequent rifting and opening of the Iapetus Ocean some 55 Myrs later.